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Solutions
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Chapter 13Properties of Solutions
Chemistry, The Central Science, 11th editionTheodore L. Brown, H. Eugene LeMay, Jr.,
and Bruce E. Bursten
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Solutions
• Solutions are homogeneous mixtures of two or more pure substances.
• In a solution, the solute is dispersed uniformly throughout the solvent.
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Solutions
The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles.
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How Does a Solution Form?
As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them.
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How Does a Solution Form
If an ionic salt is soluble in water, it is because the ion-dipole interactions are strong enough to overcome the lattice energy of the salt crystal.
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Energy Changes in Solution
• Simply put, three processes affect the energetics of solution:– separation of solute
particles,– separation of solvent
particles,– new interactions
between solute and solvent.
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Energy Changes in Solution
The enthalpy change of the overall process depends on H for each of these steps.
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Why Do Endothermic Processes Occur?
Things do not tend to occur spontaneously (i.e., without outside intervention) unless the energy of the system is lowered.
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Why Do Endothermic Processes Occur?
Yet we know the in some processes, like the dissolution of NH4NO3 in water, heat is absorbed, not released.
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Enthalpy Is Only Part of the Picture
The reason is that increasing the disorder or randomness (known as entropy) of a system tends to lower the energy of the system.
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Enthalpy Is Only Part of the Picture
So even though enthalpy may increase, the overall energy of the system can still decrease if the system becomes more disordered.
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Student, Beware!
Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved.
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Student, Beware!
• Dissolution is a physical change — you can get back the original solute by evaporating the solvent.
• If you can’t, the substance didn’t dissolve, it reacted.
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Types of Solutions
• Saturated– In a saturated solution,
the solvent holds as much solute as is possible at that temperature.
– Dissolved solute is in dynamic equilibrium with solid solute particles.
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Types of Solutions
• Unsaturated– If a solution is
unsaturated, less solute than can dissolve in the solvent at that temperature is dissolved in the solvent.
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Types of Solutions
• Supersaturated– In supersaturated solutions, the solvent holds
more solute than is normally possible at that temperature.
– These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask.
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Factors Affecting Solubility
• Chemists use the axiom “like dissolves like."– Polar substances tend to dissolve in polar solvents.– Nonpolar substances tend to dissolve in nonpolar
solvents.
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Factors Affecting Solubility
The more similar the intermolecular attractions, the more likely one substance is to be soluble in another.
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Factors Affecting Solubility
Glucose (which has hydrogen bonding) is very soluble in water, while cyclohexane (which only has dispersion forces) is not.
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Factors Affecting Solubility
• Vitamin A is soluble in nonpolar compounds (like fats).
• Vitamin C is soluble in water.
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Gases in Solution
• In general, the solubility of gases in water increases with increasing mass.
• Larger molecules have stronger dispersion forces.
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Gases in Solution
• The solubility of liquids and solids does not change appreciably with pressure.
• The solubility of a gas in a liquid is directly proportional to its pressure.
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Henry’s Law
Sg = kPg
where
• Sg is the solubility of the gas,
• k is the Henry’s Law constant for that gas in that solvent, and
• Pg is the partial pressure of the gas above the liquid.
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Temperature
Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.
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Temperature
• The opposite is true of gases.– Carbonated soft
drinks are more “bubbly” if stored in the refrigerator.
– Warm lakes have less O2 dissolved in them than cool lakes.
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Ways of Expressing
Concentrations of Solutions
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Mass Percentage
Mass % of A =mass of A in solutiontotal mass of solution
100
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Parts per Million andParts per Billion
ppm =mass of A in solutiontotal mass of solution
106
Parts per Million (ppm)
Parts per Billion (ppb)
ppb =mass of A in solutiontotal mass of solution
109
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moles of Atotal moles in solution
XA =
Mole Fraction (X)
• In some applications, one needs the mole fraction of solvent, not solute — make sure you find the quantity you need!
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mol of soluteL of solution
M =
Molarity (M)
• You will recall this concentration measure from Chapter 4.
• Since volume is temperature-dependent, molarity can change with temperature.
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mol of solutekg of solvent
m =
Molality (m)
Since both moles and mass do not change with temperature, molality (unlike molarity) is not temperature-dependent.
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Changing Molarity to Molality
If we know the density of the solution, we can calculate the molality from the molarity and vice versa.
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Colligative Properties
• Changes in colligative properties depend only on the number of solute particles present, not on the identity of the solute particles.
• Among colligative properties are– Vapor pressure lowering – Boiling point elevation– Melting point depression– Osmotic pressure
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Vapor Pressure
Because of solute-solvent intermolecular attraction, higher concentrations of nonvolatile solutes make it harder for solvent to escape to the vapor phase.
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Vapor Pressure
Therefore, the vapor pressure of a solution is lower than that of the pure solvent.
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Raoult’s Law
PA = XAPAwhere– XA is the mole fraction of compound A, and
– PA is the normal vapor pressure of A at that temperature.
NOTE: This is one of those times when you want to make sure you have the vapor pressure of the solvent.
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Boiling Point Elevation and Freezing Point Depression
Nonvolatile solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.
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Boiling Point Elevation
• The change in boiling point is proportional to the molality of the solution:
Tb = Kb m
where Kb is the molal boiling point elevation constant, a property of the solvent.Tb is added to the normal
boiling point of the solvent.
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Boiling Point Elevation
• The change in freezing point can be found similarly:
Tf = Kf m
• Here Kf is the molal freezing point depression constant of the solvent.
Tf is subtracted from the normal boiling point of the solvent.
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Boiling Point Elevation and Freezing Point Depression
Note that in both equations, T does not depend on what the solute is, but only on how many particles are dissolved.
Tb = Kb m
Tf = Kf m
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Colligative Properties of Electrolytes
Since these properties depend on the number of particles dissolved, solutions of electrolytes (which dissociate in solution) should show greater changes than those of nonelectrolytes.
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Colligative Properties of Electrolytes
However, a 1M solution of NaCl does not show twice the change in freezing point that a 1M solution of methanol does.
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van’t Hoff Factor
One mole of NaCl in water does not really give rise to two moles of ions.
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van’t Hoff Factor
Some Na+ and Cl- reassociate for a short time, so the true concentration of particles is somewhat less than two times the concentration of NaCl.
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van’t Hoff Factor
• Reassociation is more likely at higher concentration.
• Therefore, the number of particles present is concentration-dependent.
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van’t Hoff Factor
• We modify the previous equations by multiplying by the van’t Hoff factor, i.
Tf = Kf m i
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Osmosis
• Some substances form semipermeable membranes, allowing some smaller particles to pass through, but blocking other larger particles.
• In biological systems, most semipermeable membranes allow water to pass through, but solutes are not free to do so.
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Osmosis
In osmosis, there is net movement of solvent from the area of higher solvent concentration (lower solute concentration) to the are of lower solvent concentration (higher solute concentration).
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Osmotic Pressure
The pressure required to stop osmosis, known as osmotic pressure, , is
nV
= ( )RT = MRT
where M is the molarity of the solution.
If the osmotic pressure is the same on both sides of a membrane (i.e., the concentrations are the same), the solutions are isotonic.
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Osmosis in Blood Cells
• If the solute concentration outside the cell is greater than that inside the cell, the solution is hypertonic.
• Water will flow out of the cell, and crenation results.
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Osmosis in Cells
• If the solute concentration outside the cell is less than that inside the cell, the solution is hypotonic.
• Water will flow into the cell, and hemolysis results.
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Colloids
Suspensions of particles larger than individual ions or molecules, but too small to be settled out by gravity are called colloids.
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Tyndall Effect
• Colloidal suspensions can scatter rays of light.
• This phenomenon is known as the Tyndall effect.
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Colloids in Biological Systems
Some molecules have a polar, hydrophilic (water-loving) end and a non-polar, hydrophobic (water-hating) end.
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Colloids in Biological Systems
Sodium stearate is one example of such a molecule.
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Solutions
© 2009, Prentice-Hall, Inc.
Colloids in Biological Systems
These molecules can aid in the emulsification of fats and oils in aqueous solutions.